Variable gain amplifiers (VGAS) are frequently used in modern radio receivers to amplify or attenuate incoming signals to properly drive an associated analog-to-digital converter (A/D). Typically, the variable gain is distributed among radio frequency (RF), intermediate frequency (IF), and low-frequency or baseband circuits. Because the RF front-end low noise amplifier (LNA) must detect weak signals in the presence of strong interfering signals, it must have both a low noise figure, to increase the sensitivity of the receiver, and high linearity to prevent interference from unwanted adjacent channel signals. As the desired input signal strength rises above sensitivity, reducing the LNA gain increases the dynamic range of the receiver and eases the linearity requirements of the downconverting mixer and following stages in the receiver chain.
Several types of LNA architectures have been used in an effort to meet the performance requirements of modern radio receivers. For example, in some typical LNA architectures, the LNA gain control has conventionally been implemented in discrete steps. In it's most basic form, a step-gain LNA consists of two amplifiers in parallel, one having the desired low noise and high gain and the other having the desired low gain and high input overload capability. However, one significant problem associated with this implementation is that high noise levels are experienced at low gain.
Additionally, step-gain LNAs have several other drawbacks. First, because the step LNA does not provide for a smooth transition between gain states, it does not allow the optimization of the operating characteristics of the overall system under all operating conditions. With the LNA in a given fixed-gain state, as the input power is increased, the linearity of the following stages is used up. To prevent the system from falling out of operating specifications before the LNA is switched to its next gain state, sufficient margin, at the expense of increased current consumption, must be added to the linearity of the following stages. Second, the gain of the LNA and the IF VGA must be controlled in a seamless fashion. In some wireless communications systems, such as Code Division Multiple Access (CDMA), the gain of the IF VGA is adjusted continuously by an analog control voltage. With a multi-step LNA, separate digital control signals as well as circuitry to generate the required switching hystersis characteristic are needed, which add complexity and consume additional chip area. Third, a step-gain LNA introduces transient glitches during gain switching that could possibly distort the amplitude and phase of the signal waveform.
One way to address the problems associated with the step-gain LNA is to use a continuously variable gain LNA. An LNA with continuously variable gain overcomes the aforementioned shortcomings since it may allow the performance of the following stages to be made close to the minimum required specifications, can be controlled with the same AGC control voltage as used with the IF VGA, and minimizes the generation of transients as the gain is varied. A number of IF amplifiers with continuous gain control have been developed. However, conventional circuits of this type suffer from two well-known disadvantages. First, under high attenuation (when a large overload capability is required in a receiver) the amplifier's linearity is severely reduced. Second, similarly to the current-splitting step LNA, the noise figure of the continuous current-splitting amplifier degrades significantly at low gains, because in these circuits most of the signal current has been dumped to the power supply.
Accordingly, a need remains for a continuous variable-gain LNA that overcomes the disadvantages of current techniques, while meeting all the stringent performance requirements of an LNA for use in a modem radio receiver.